A shooting optical system used for a camera such as a video camera or a digital still camera having a solid-state image pickup element (e.g., CCD or CMOS image sensor) is required to be a zoom lens system having a high zoom ratio and high optical performance.
As one of the zoom lens systems that can meet the requirements, there is known a positive lead zoom lens system in which a lens unit having a positive refractive power is disposed at the object side.
The positive lead zoom lens system can be easily adapted to have a high zoom ratio, and hence is often used for a zoom lens system having a zoom ratio of 10 or higher, in particular. For instance, there is known a zoom lens system including five lens units having positive, negative, positive, negative and positive refractive powers in this order from the object side to the image side (see Japanese Patent Application Laid-Open No. 2004-240398 corresponding to U.S. Patent Application No. 2006/0152816, Japanese Patent Application Laid-Open No. H05-215967 (corresponding to U.S. Pat. No. 5,388,004), Japanese Patent Application Laid-Open No. 2001-330777 (corresponding to U.S. Pat. No. 6,631,034), Japanese Patent Application Laid-Open No. 2002-365547, and Japanese Patent Application Laid-Open No. S49-066354 (corresponding to U.S. Pat. No. 3,912,373).
In general, to realize a high zoom ratio in a zoom lens system, a refractive power of the lens unit that changes a total zoom ratio of the entire zoom lens system (hereinafter referred to as a principal magnification lens unit) should be enhanced, or a movement amount of the principal magnification lens unit for zooming should be increased mostly.
However, a high zoom ratio can be realized if the refractive power of the principal magnification lens unit is simply enhanced so as to increase the movement amount, but an aberration variation upon zooming increases so that it becomes difficult to obtain high optical performance over the entire zoom range.
In order to realize a high zoom ratio and to obtain high optical performance over the entire zoom range, it is important to set magnification sharing of individual lens units of the zoom lens system for zooming appropriately and to set the movement amount appropriately.